It is generally known that security elements may be used for security papers and security items in general, for example, for bank notes, checks, stocks and bonds, securities, identification cards, passports, drivers licenses, admission tickets, stamps and similar documents or for bank cards, credit cards and the like, for the purpose of said security elements is to prevent or interfere with counterfeiting of these objects by unauthorized parties (R. van Renesse, Optical Document Security (1997), Artech House, Boston). Likewise, such security elements are used to identify the validity or authenticity of objects or to permit or facilitate identification of objects in general.
For example, it is a widespread practice to use security threads or strips which may consist of a metal-coated plastic, for example, in security papers, in particular for use in bank notes and similar valuable papers. If these security threads or strips are embedded in the security paper, for example, which is subsequently printed at any rate, these security elements cannot readily be seen when the object is observed in reflection. However, when light is passed through the object, these security elements appear as a dark shadow and therefore can be observed in transmission. In particular to guarantee protection against counterfeiting of security items such as security papers, many proposals have been made in recent times for providing security elements with certain properties so that not only the presence of security elements per se but also in particular the presence of specific properties should guarantee the authenticity of the object thus secured (U.S. Pat. No. 4,897,300; U.S. Pat. No. 5,118,349; U.S. Pat. No. 5,314,739; U.S. Pat. No. 5,388,862; U.S. Pat. No. 5,465,301; German Patent Application 1,446,851; British Patent 1,095,286). For example, a security thread having a multicolor micro-printing is known from German Patent Application 1,446,851; the printing ink may also be fluorescent in this case. The surfaces printed with different colors are so small or so close together in the case of this thread that they can hardly be differentiated with the naked eye and therefore appear to the observer as a single-color pattern. However, the micro-printing and the different colors can be observed with the help of a magnifying glass or a microscope. A similar security element is described in British Patent 1,095,286, where the micro-printing claimed in the aforementioned patent specification consists of characters and patterns. U.S. Pat. No. 4,897,300, however, describes a security paper in which are embedded several security threads which are printed with different luminescent dyes. In the unexcited state, the latter are colorless or the same color as the paper and therefore are difficult or impossible for the observer to see. Due to excitation, e.g., irradiation with ultraviolet (UV) light, the security threads assume a luminescence of an extent that permits detection by the naked eye. In addition, characteristic mixed colors are formed by overlapping of security threads of different colors. To further increase the security of security papers, especially bank notes, a security thread or strip of plastic is also integrated into the paper so that the “window” in the paper surface allows a direct view of parts of the surface of the security element, as described in British Patent 1,552,853, British Patent 1, 604, 463 or European Patent 0,059,056, for example.
However, one serious disadvantage of all these known security elements is that either the characteristic authenticity features are relatively difficult for a layperson to discern or they require complex equipment for detection, or on the other hand, an easily detectable authenticity feature may be counterfeited relatively easily. However, it is in the nature of the matter that security items are often replaced by newer products with newer security elements after a comparatively short period of time, in particular to prevent counterfeiting and other forms of abuse. Therefore, there has been an urgent need for new, secure and easily discernible security elements for security papers and for security items in general.
One solution that is interesting in this regard has already been described in International Patent Application WO-A-98 01817, where photoluminescence polarizers, where the emitted luminescence light is linearly polarized, are used as security elements for security objects. When this light is observed through a polarizer and the polarizer is rotated, the result for the observer is a light-dark effect because of the luminescence light which is either transmitted or not transmitted through the polarizer, depending on the rotational position. Although U.S. Pat. No. 5,284,364 uses polarizers for security purposes, they are “ordinary” polarizers such as λ/2 layers and are not photoluminescence polarizers.